This research investigated the effectiveness of jury directions in criminal trials. Jury directions instruct the jury about the relevant law and how the law should be applied to the evidence in a trial. Two studies were conducted to investigate whether simplifying current jury directions increases mock jurors’ ability to comprehend and apply those directions. In both studies, mock jurors listened to a simulated criminal trial, provided a verdict and completed a questionnaire. Study 1 manipulated type of jury direction (simple, standard or summary) and evidence strength (weak or strong) in a between-subjects design to assess mock jurors’ comprehension and application of the directions. The results of 108 mock jurors indicated that a simple direction was easier to comprehend than a standard or summary direction. There was some evidence to suggest that mock jurors applied a simple direction better than a standard or summary direction. Study 2 manipulated type of jury direction (simple or standard) and cognitive load (low or high) in a between-subjects design to examine mock jurors’ application of the directions under high cognitive load. The results of 72 mock jurors replicated the findings in Study 1 for comprehension and self-reported application. The effect of cognitive load on mock jurors’ ability to apply the directions could not be examined because the cognitive load manipulation was not successful. Both studies suggest that simplifying current jury directions enhances mock jurors’ comprehension of the directions. Further research is required to investigate whether mock jurors are able to apply jury directions, specifically under high cognitive load.